Process for recovering chromic anhydride from exhausted aqueous chromium plating bath solutions with exploitation of the recovered chromium

ABSTRACT

The chromic anhydride contained in soluble form (chromic acid) in exhausted, polluted chromium plating baths is recovered by alkalizing the exhausted chromium plating liquid to a pH comprised between 3 and 7 at a temperature of 70° C. with a commercial sodium hydroxide solution, keeping the mass in reaction at this temperature for an hour and filtration separating the precipitate obtained. The filtered solution having a specific gravity of 1.25 kg/l, contains CrO 3  (22% in weight) which is then reduced with sulphur dioxide to obtain a solution containing basic chromium sulphate and sodium sulphate, suitable for use as a tanning bath; in this manner pollution is avoided and the waste chromium is exploited. The solution containing CrO 3  can also be introduced into a hexavalent chromium salt production cycle.

The subject of the present invention is a process for recoveringchromium anhydride contained in soluble form (chromic acid) in exhaustedchromium plating baths polluted by the presence of various metallicions, particularly iron, copper, zinc, aluminium, nickel, cadmium, aswell as by the appearance of trivalent chromium, which forms duringchromium plating.

What is meant by exhausted solution is a chromium plating bath that canno longer be used for this purpose because it is polluted with metalions (essentially iron, copper, zinc, aluminium, nickel, cadmium) and bythe appearance of trivalent chronium. A typical example of a so-calledexhausted chromium plating bath has the following composition:

    ______________________________________                                        CrO.sub.3 (total) =                                                                         35%    (weight/weight)                                          CrO.sub.3 (hexavalent) =                                                                    30%    (weight/weight)                                          CrO.sub.3 (trivalent) =                                                                     5%     (weight/weight)                                          Fe =          30     parts per 1000 parts of total CrO.sub.3                  Cu =          30     parts per 1000 parts of total CrO.sub.3                  Zn =          5      parts per 1000 parts of total CrO.sub.3                  Al =          1      part per 1000 parts of total CrO.sub.3                   Ni =          0.1    parts per 1000 parts of total CrO.sub.3                  V =           0.05   parts per 1000 parts of total CrO.sub.3                  Cd =          0.2    parts per 1000 parts of total CrO.sub.3                  ______________________________________                                    

Depending on the chromium plating processes from which the baths come,the following may also be present: Ca, Mg, Si, Na, SO₄, Cl⁻. There mayalso be present chinonic organic substances whose disruptive action mayeasily be stopped by adding mercaptoacetic acid to the chromium sulphateobtained (as described in Italian patent application No. 21807 A/82).

A known method is that of precipitating metallic hydrates contained inaqueous solutions, at controlled pH values, and separating them on thebasis of their relative separation factors, see Chemical Economy andEngineering Review; May 1975, vol. 7 No. 5 (No. 84) and references.

Another known method is that of extracting chromic anhydride fromaqueous solutions where it is found together with polluting metallicions, by means of ion exchange resins in solid or liquid form and withappropriate solvents (French patent application No. 2,179,649). Alsoknown is the method (U.S. Pat. No. 4,045,340) consisting ofprecipitating the internal salt of hexavalent and trivalent chromium,known as chromium chromate, in the cycle of sodium bisulphate recoveryfrom a chromium anhydride synthesis plant.

Now it has surprisingly been discovered that at certain pH values it ispossible to precipate simultaneously metallic ions (which pollute anexhausted chromium plating bath) and chromium chromate, which is, asmentioned earlier, an internal salt of hexavalent chromium and trivalentchromium, which are present in the bath.

To be precise, it has been found in particular that the liquidsseparated by percolation from the chromium chromate precipitate, aresuitable for use in a hexavalent chromium salt manufacturing process orto be transformed, by reduction of hexavalent and trivalent chroumiumwith sulphur dioxide, into a basic chromium sulphate suitable for use intanning hides, as will be described in greater detail further on, on thebasis of illustrative examples.

With this transformation, materials which, being toxic and harmful,would cause serious ecological problems if they were released into theenvironment, are recovered in the form of products of considerableeconomic and commerical value, with a much reduced effect on theenvironmental preservation.

In fact disposal problems are reduced to about 10% and the processclaimed herein therefore proves generally more economical.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically compares the relative curves of the titration at roomtemperature of a typical chromium plating bath with NaOH and the sametitration carried out on a synthetically obtained solution, with onlychromic anhydride dissolved in water, at the same concentration andtemperature.

FIG. 2 graphically illustrates the separation factor pattern for iron,copper and trivalent chromium from hexavalent chromium at different pHvalues.

DETAILED DESCRIPTION OF THE INVENTION

During the studies leading to this discovery, basic chromium sulphate ina residual liquid from chromium plating baths polluted with Fe, Cu, Ni,Al, Zn, V and trivalent chromium was at first transformed directly,reducing the chronium anhydride with sodium bisulphite.

However it was noted that the resulting liquid, used as a tanning bath,gave unsatisfactory results for this operation; it was therefore deducedthat a preliminary operation was needed to purify the exhausted,polluted chromium plating bath.

To this end a typical chromium plating bath as described above wastitred at room temperature with NaOH and the relative curve (pH vs cc ofN/10 NaOH used) was plotted (see FIG. 1) compared with the sametitration (and relative curve) carried out on a synthetically obtainedsolution, with only chromic anhydride dissolved in water, at the sameconcentration.

An anomaly was observed in the titration curve between the pH values(experimental and theoretical) from 2 to 5.5. In fact in the chromiumplating bath at pH 2 solid precipitation began and the pH valuesubsequently rose more rapidly (up to a value of 5.5) in contrast withwhat happened in the synthetic solution.

From pH 5.5 to 10 (which is the point at which bichromate is completelytransformed to monochromate) behavior was the same in both cases.

The above is schematized in the attached graph (FIG. 1), from which itcan be noted that in both cases the following reaction takes place alongline A:

    2HCrO.sub.4.sup.- →Cr.sub.2 O.sub.7.sup.-2 +H.sub.2 OA

whereas along B (synthetic solution) the previous reaction continues andalong B₁ (chromium plating bath) the hydrate precipitation reactionscombine and predominantly the reaction:

    HCrO.sub.4.sup.- +2Cr.sup.+3 +50H.sup.- chromium chromate precipitate CrO.sub.3.Cr.sub.2 O.sub.3 +3H.sub.2 O

which essentially determines the anomaly shown by curve B₁ compared withB. At a pH above 5.5 the following reaction along line C takes place inboth cases:

    Cr.sub.2 O.sub.7.sup.-2 +20H.sup.- →2CrO.sub.4.sup.-2 --H.sub.2 O

until completion (at pH 10) of the transformation to monochromate.

It was thus discovered that by alkalizing the residual liquid fromchromium plating baths, polluted with Fe, Cu, Ni, Al, Zn, V, Cd andCr^(III), with sodium hydroxide added in such an amount as to bring thepH value to between 3 and 7, all metallic pollutants, presentprecipitate: surprisingly, by subsequently filtering and subjecting thefiltrate (containing Cr^(VI)) to the action of SO₂, a liquid is obtainedcontaining basic chromium sulphate and sodium sulphate, which has provedsuitable, without further modification, for use in tanning baths withexcellent results.

The operation described above was repeated several times, filtering theliquids brought to pH values between 3 and 7, and chromium sulphatessuitable for hide tanning were always obtained, even though theypresented different basicity values (ratio between SO₃ linked to thechromium and chromium itself) according to the pH value selected forprecipitation.

The separation factor pattern for iron, copper and trivalent chromiumfrom hexavalent chromium was also examined at different pH values andthe results are set out in FIG. 2 (in which the data are complete up topH 5 for all elements).

In this figure it can be seen clearly that in the pH interval from 3 to7, an interval of special importance as we shall see later, for reasonsdiscussed further on, the separation factors are such that their valuesdo not subsequently interfere with hide tanning operations, as emergesclearly from the examples.

At pH 3 the filtered liquids contain a solution of sodium bichromateaccording to completion of the reaction:

    2HCrO.sub.4.sup.- →Cr.sub.2 O.sub.7.sup.-2 +H.sub.2 O (curve A in FIG. 1)

At pH 7 the filtered liquids contain hexavalent chromium, 50% of whichis in the form of a bichromate and 50% in the form of a monochromateaccording to partial occurrence of the reaction:

    Cr.sub.2 O.sub.7.sup.-2 +20H.sup.- →2CrO.sub.4.sup.-2 +H.sub.2 O (curve C in FIG. 1)

By reducing the hexavalent chromium in the liquids filtered at pH 3 thefollowing is obtained:

    NaCr.sub.2 O.sub.7 +3SO.sub.2 +H.sub.2 O→Na.sub.2 SO.sub.4 +2(CrOHSO.sub.4)

which is the so-called Schorlemmer 33.3% basicity basic chromiumsulphate, which is used in certain types of hide tanning.

By reducing the hexavalent chromium with SO₂ in the liquids filtered atpH 7 the following is obtained:

    0.5 Na.sub.2 Cr.sub.2 O.sub.7 +Na.sub.2 CrO.sub.4 +3SO.sub.2 +1.5 H.sub.2 O→1,5 Na.sub.2 SO.sub.4 +2[Cr(SO.sub.4).sub.0,75 (OH).sub.1.5 ]

The latter is the so-called 50.00 Schorlemmer basicity basic chromiumsulphate, which is used in certain other particular types of hidetanning.

Proceeding as described above at intermediate pH values (for example5-6) intermediate basic chromium sulphates are obtained.

The aim of the present invention is therefore to obtain from chromiumplating residual materials which, according to present laws, areclassified as toxic and harmful, products of economic value, such asbichromates, chromates, chromium sulphates and derivatives thereof.

In other words, in practice the process according to the presentinvention on the one hand solves the ecological problem of preservingthe environment from toxic substances and on the other hand offers theadvantage of simultaneously exploiting what is at present a dangerouswaste product.

The process described can be modified, without altering its substance,with a preliminary operation to oxidize the trivalent chromium containedin exhausted chromium plating baths to hexavalent chromium, according toknown methods.

This operation reduces from about 10% to about 1% (in the case of thetypical example shown for the composition of a chromium plating bath)the onus deriving from the need to dispose of solid waste, and thechromic anhydride obtained from the oxidation operation is recovered.

EXAMPLE 1 Processing of Exhausted Chromium Plating Liquids According toa known process (chromium reduction without preliminary purification)

To an exhausted liquid (10 kg), from surface chromium platingoperations, having the composition indicated in the description, a 36%solution of sodium bisulphite was added, with no preliminary operation,until the hexavalent chromium had disappeared. The resulting liquid,whose physical appearance was similar to that indicated in example 2,had the following characteristics:

    ______________________________________                                        Cr.sub.2 O.sub.3 content =                                                                   12,2%   (16,0% expressed in CrO.sub.3)                         Schorlemmer basicity =                                                                       34%                                                            Fe content =   480     mg per 100 g of liquid                                 Cu content =   480     mg per 100 g of liquid                                 Ni content =   1,6     mg per 100 g of liquid                                 Al content =   16      mg per 100 g of liquid                                 Cd content =   25      mg per 100 g of liquid                                 Zn content =   80      mg per 100 g of liquid                                 V content =    0.3     mg per 100 g of liquid                                 ______________________________________                                    

This material was used for a hide tanning test, as will be describedfurther on in example 7.

EXAMPLE 2 Processing Exhausted Liquids According to the Invention

An exhausted liquid (10 kg) from surface chromium plating operations,having the composition indicated in the description, was brought to pH5, from an initial pH of 0.7, with a commercial sodium hydroxidesolution, NaOH titre 45% (2.5 kg).

After the sodium hydroxide had been added, the temperature had risen to70° C. The resulting mass was kept at 70° C. for 1 h, in order to aidsettling of the precipitated solid, and then filtered to obtain:

a solid which, after drying at 110° C., weighed 1 kg, and

a liquid with a specific gravity of 1.25 kg/1 having a CrO₃concentration of 22% in weight.

The CrO₃ recovered from the filtered liquid was equivalent to 75% ofthat contained in the exhausted liquid.

This liquid, purified and filtered, can be used for the synthesis ofbichromate and monochromate. In the specific case of the presentexample, it was subjected to the reducing action of SO₂, which wasbubbled through the liquid until the hexavalent chromium haddisappeared, and a solution of basic chromium and sodium sulphate havingthe following characteristics was obtained:

    ______________________________________                                        Cr.sub.2 O.sub.3 content =                                                                   13.3%   (17.5% expressed in CrO.sub.3)                         Schorlemmer basicity =                                                                       35%                                                            Fe content =   1.6     mg per 100 g of liquid                                 Cu content =   1.5     mg per 100 g of liquid                                 Ni content =   0.7     mg per 100 g of liquid                                 Al content =   0.5     mg per 100 g of liquid                                 Zn, V and Cd content =                                                                       0.1/0.2 mg per 100 g of liquid                                 ______________________________________                                    

A hide tanning test was carried out with this material, as described inexample 6.

EXAMPLE 3 Processing Exhausted Chromium Plating Liquids According to aVariant of the Invention

10 kg of an exhausted chromium plating bath, having the compositionindicated in the description, were brought to pH 7 (from an initialvalue of 0.7) with a commercial sodium hydroxide solution, NaOH strength45%; the amount of sodium hydroxide used was 4 kg. After the sodiumhydroxide had been added the temperature had risen to 60° C. theresulting mass was kept under slow agitation for 1 h at thistemperature, in order to aid settling of the precipitated solid thenfiltered to obtain:

a solid which, after drying at 110° C. to a constant weight, weighed 0.9kg and

a liquid having a specific gravity of 1,4 kg/liter and a CrO₃concentration of 21% in weight.

The CrO₃ separated from the liquid by filtration (compared with thatcontained in the exhausted liquid) was equivalent to 78%. This liquidcould have been used for the synthesis of bichromate and/ormonochromate, but, for closer comparison, it was subjected, as describedin example 1, to the action of SO₂ until the hexavalent chromium haddisappeared; a chromium sulphate and sodium sulphate solution having thefollowing characteristics was thus obtained:

    ______________________________________                                        Cr.sub.2 O.sub.3 content =                                                                    12.2    (16% expressed in CrO.sub.3)                          Schorlemmer basicity =                                                                        50,5%                                                         Fe content =    1.0     mg per 100 g of liquid                                Cu content =    0.6     mg per 100 g of liquid                                Ni content =    0.3     mg per 100 g of liquid                                Al content =    0.2     mg per 100 g of liquid                                Zn, V and Cd content =                                                                        0.1/0.2 mg per 100 g of liquid                                ______________________________________                                    

The material thus obtained was used to carry out a hide tanning test asdescribed in example 8.

EXAMPLE 4 Processing of exhausted liquids according to a further variantof the invention

An exhausted chromium plating bath having the following composition wasused:

    ______________________________________                                        CrO.sub.3 (total) =                                                                        18.5%   (weight/weight)                                          CrO.sub.3 Hexavalent =                                                                     17.5%   (weight/weight)                                          CrO.sub.3 Trivalent =                                                                      1.0%    (weight/weight)                                          Fe =         50      parts per 1000 parts of total CrO.sub.3                  Cu =         30      parts per 1000 parts of total CrO.sub.3                  Zn =         5       parts per 1000 parts of total CrO.sub.3                  Al =         1       part per 1000 parts of total CrO.sub.3                   Ni =         0.1     parts per 1000 parts of total CrO.sub.3                  V =                  traces                                                   ______________________________________                                    

and the following operations were performed:

100 kg of a bath having an initial pH of 0.5 were brought to pH 4 with asodium hydroxide solution having NaOH titer 45% (15 kg). After agitationfor 1 h at 60° C. it was filtered to obtain:

15 kg of wet solid (loss at 110° C.=50%) =7.5 kg dry

100 kg of liquid having the following composition:

    ______________________________________                                        Hexavalent CrO.sub.3 =                                                                         15%    (weight/weight)                                       Trivalent CrO.sub.3 =   absent                                                Fe =             2      mg per 100 g of liquid                                Cu =             2      mg per 100 g of liquid                                Zn =             0,2    mg per 100 g of liquid                                Al =             1      mg per 100 g of liquid                                Ni =             1      mg per 100 g of liquid                                V =                     traces                                                ______________________________________                                    

This liquid was concentrated by evaporating the water to a CrO₃concentration of 50% (weight/weight) and used to obtain both directly,by crystallization, sodium bichromate (Na₂ CrO₇ .2H₂ O) and, by additionof alkalis (sodium hydrate) and subsequent crystallization, sodiummonochromate (Na₂ CrO₄. 4H₂ O).

EXAMPLE 5 Processing Exhausted Liquids According to a Further Variant ofthe Invention

To 10 kg of an exhausted chromium plating bath, having the compositionindicated in the description, sodium persulphate was preliminarily addeduntil the trivalent chromium had disappeared. The bath was then treatedas described in example 2.

A solid weighing 0.1 kg after drying at 110° C. and a liquid whollysimilar in its characteristics to that in example 2 were obtained.

This liquid, thus purified and filtered, can be used for the synthesisof bichromate, monochromate and chromium sulphate, as described in allpreceding examples.

The tanning operations, described in greater detail in the exampleswhich follow, are carried out using:

A) as laboratory apparatus:

a 70 cm diameter drum with a rotation speed of 10 rpm.

B) as tannery apparatus:

a 2500 cm diameter drum with a rotation speed of 8 rpm.

C) as material for processing:

light calf, kid or heavy cow skins ready for tanning pickled at pH 2,8,with a beamhouse weight of 30 kg for testing in the laboratory drum and1500 kg for testing in the semi-industrial drum.

In all the examples that follow all percentages refer to skins ready fortanning.

EXAMPLE 6 Tanning with 35 Sch Basicity--Cr₂ O₃ Titre 13.3--ConductedAccording to the Present Invention.

After a successful test carried out in a laboratory drum, tanning wasrepeated in a semi-industrial drum.

1500 kg of cow hides (each weighing 40-45 kg) were loaded into a drumwith 100% water and 5% NaCl and after rotation for 10 minutes, 0.5% ofconcentrated H₂ SO₄ was added. The drum was then kept in rotation for 30minutes and the pH of the liquid when checked afterwards proved to be3.6.

A further 0.25% of concentrated H₂ SO₄ was added and, after rotation for20 minutes, a sample of the liquid was removed and its pH proved to be2.7, while the hide section had a pH of between 2.7 at the outside and 3in the middle.

The liquid was drained from the drum and 90% water and 9% basic chromiumsulphate solution, containing 13.3% of Cr₂ O₃ and having 35 Schorlemmerbasicity, were added (the procedure for obtaining this solution wasdescribed in example 1). After rotation for 10 minutes 1% of stuffingwas added and the drum was kept rotating for 1 h.

0.5% of sodium formate was subsequently added. After rotation for 3 hthe hide was checked for reaction and found to be completely crossed bythe chromium. The temperature of the bath was 38° to 40° C.

Basification was then begun by adding, in three stages, NaHCO₃ andprecisely 0.3% at each stage, and rotating the drum for 20 minutesbetween the first and second addition. When all the sodium bicarbonatehad been added, the drum was kept in rotation; the pH was checked after1 h and found to be 4.2.

The hides were then treated against mold so that they could be kept atlength in wet blue and after the drum had been rotated for 1 h, thehides were removed and left in piles for 48 h before pressing, splittingand shaving. Treated as described, the hides showed a uniform tanningcolor and even gelling temperature (GT). After the hides had been storedboth the split and the grain were examined again and no abnormalitieswere found: they had remained of a uniform colour with good softness andfullness characteristics.

EXAMPLE 7 Tanning with 34 Sch Basicity--12.2 Titre

The 8/12 kg pickled hides of 3 French calves (beamhouse weight 30 kg)were placed in a laboratory tanning container (drum) and treated with80% water, 2.6% Cr₂ O₃ from a solution of basic chromium sulphate,having the following characteristics:

    ______________________________________                                        basicity        34%    Schorlemmer                                            Fe content =    480    mg per 100 g of liquid                                 Cu content =    480    mg per 100 g of liquid                                 Ni content =    1.6    mg per 100 g of liquid                                 Al content =    16     mg per 100 g of liquid                                 Zn content =    80     mg per 100 g of liquid                                 V content =     0.8    mg per 100 g of liquid                                 ______________________________________                                    

After rotation for 2 h the chromium was found to have crossed the entirehide section.

Three per cent of a 10% sodium carbonate solution was then added to thebath.

Rotation was continued for 30 minutes, the a further 3% of 10% sodiumcarbonate solution was added and rotation was continued for 30 minutes,after which a further 4% of 10% sodium carbonate solution was added;rotation was continued for 6 h. At the end of the six hours the pH ofthe solution was 3.8. The hides were unloaded from the drum, piled upfor 48 h and then checked for appearance before undergoing furtherprocessing. An abnormal clouring was immediately evident (dark gree,tending to brown, with patches distributed over the whole surface).

The cause was put down to the impurities contained in the liquid,particularly to the presence of iron and copper which, as is known, areharmful to tanning operations.

EXAMPLE 8 Tanning According to the Invention with 50 SchorlemmerBasicity and 12.2% Cr₂ O₃

After deliming, soaking and pickling, 1500 kg of cow hides were placedin a semi-industrial drum with 1.8% of the pickling bath solution inorder to extablish a pH of 3.5.

To this was added 7% of 12.2% Cr₂ O₃ solution with a basicity of 50degrees Schorlemmer taken from the exhausted chromium plating bathsolution purified according to the method claimed in the presentinvention. The drum was kept in rotation for 30 minutes; stuffing wasadded in two stages and the drum was kept rotating for 2 h. A further 4%of 12.2% Cr₂ O₃ solution with 50° Schorlemmer basicity (from recovery ofchromium plating baths) was added and, after checking the pH of thebath, which proved to be 3.8, the hides were removed and left piled upfor 48 h.

The technital data confirmed a positive tanning result, with a goodcolour, regular penetration speed, absence of patches after 10 days ofwet storage, even grain. The hides were subsequently pressed, split andshaved to 1.2 mm, then neutralizing, dying and stuffing was carried out.

The hides, subsequently pressed, were dried and perched. A good uniformcolour, the same degree of fullness and softness in all skins and goodblowing strength were found.

We claim:
 1. A process for recovering chromic anhydride from exhaustedchromium plating baths polluted by metal compounds, comprising the stepsof:a) adding an alkali to the exhausted solution to adjust it to a pHrange between 3 and 7; b) precipitating the metallic pollutants; and c)separating the precipitate obtained in step b) from the solutioncontaining hexavalent chromium by filtration, wherein the chromium ofsaid solution are partially oxidized from the trivalent to thehexavalent state and said metallic pollutants are coprecipitatedtogether with chromium chromate.
 2. The process of claim 1, wherein saidcoprecipitation is carried out at a pH within the range from 5 to
 6. 3.The process of claim 1, wherein the filtered solution containinghexavalent chromium is subjected to the action of a reducer.
 4. Theprocess of claim 3, wherein the reducer is sulfur dioxide which producesa solution of basic chromium sulfate and sodium sulfate to be used as ahide tanning bath.
 5. The process of claim 1, wherein the alkali iscommercial sodium hydroxide.
 6. The process of claim 1, wherein thefiltered solution containing hexavalent chromium is then used withoutfurther purification as a basic material for the synthesis of hexavalentchromium salts.